1. Field of the Invention
The present invention relates to a vacuum vessel suitable for obtaining an ultrahigh vacuum or an extremely high vacuum needed in semiconductor manufacturing apparatus and in particle accelerators, and a method for manufacturing thereof.
2. Description of the Related Art
In manufacturing a semiconductor device having a very high integration density, even a minute defect at the time of a thin film formation process will result in definite damage in the performance of the device. Therefore, the need arises for an extremely high vacuum, which is higher than an ultrahigh vacuum, in a thin film deposition apparatus for the purpose of preventing contamination in the grating lattice due to foreign elements as well as the introduction of minute dust particles which may cause defects in the semiconductor device.
Realization of an extremely high vacuum is indispensable not only in the semiconductor field but also in the field of particle accelerators used in nuclear fusion reactors for the purpose of maintaining a long lifetime of accelerated particles. Research for achieving extremely high vacuum is under study in various fields.
In order to produce ultrahigh vacuum or extremely high vacuum, an evacuating system that can achieve a lower pressure and that has a large exhaust capacity is required. Suppressing the generation of gas or off-gassing from the inner wall of a vacuum vessel and prevention of leakage from the joints of the vacuum vessel are particularly important factors for attaining and maintaining ultrahigh vacuum or extremely high vacuum.
The wall of a conventional vacuum vessel is formed of stainless steel or aluminum alloy. A vacuum vessel formed mainly of such materials exhibits a great amount of gas generation or off-gassing from the metal surfaces and also from the inside of the metal walls during evacuation. The main component of the generated gas is water vapor at relatively low vacuum levels where baking is not carried out, and is hydrogen when baking is carried out and water removed. Although the amount of gas generation can be reduced by raising the baking temperature, the baking temperature of a metal vessel is limited to approximately 300.degree. C. It was therefore considered impossible to completely suppress gas generation by baking.
Various methods for suppressing gas generation other than by baking have been considered, such as using stainless steel of low hydrogen occlusion manufactured by dissolving a metal material of low impurities under vacuum, processing the inner wall of an aluminum alloy by discharging in a gas mixture of argon and oxygen to form an oxide film on the aluminum alloy, or a combination of these methods and also applying a mirror-finish to the inner wall formed of stainless steel or aluminum alloy. The gas generation can be reduced considerably by combining these methods and baking. It has been reported that an extremely high vacuum on the order of 10.sup.-13 Torr was obtained with a vacuum vessel made of stainless steel or aluminum alloy. However, generation of hydrogen gas was exhibited from the wall of such vessels, so that the vacuum that could eventually be obtained was limited by the hydrogen gas. The development of a vacuum vessel with extremely low gas generation is desired.
In the field of a particle accelerator, an electric field or a magnetic field is applied in the vacuum vessel for controlling the motion of the charged particles. In the present state of the art where the coil for generating an electromagnetic field is provided outside of the vacuum vessel, a vacuum vessel formed of either stainless steel or aluminum alloy, which have the effect of shielding the magnetic field and the electric field, has the problem of disabling the high precision control of the accelerated particles. It has been impossible to form a vacuum vessel accommodating a coil to solve this problem because of limitations associated with materials and shapes of the vessel.
An approach using a vacuum vessel made of glass that has a low hydrogen occlusion and that easily passes electric field and magnetic field for the precise control of accelerated particles could be considered. However, the vessel will have a low reliability due to the forces exerted on the wall of the vessel during evacuation, because the strength of glass is low and it is easily broken. Furthermore, glass begins to soften during baking, or the glass may crack due to thermal stress caused by any nonuniformity of the baking temperature. Therefore, glass is not practical to be used to make a vacuum vessel.